1. Field of the Invention
The present invention relates to a fall detection apparatus more specifically, an apparatus that detects a fall by detecting acceleration.
2. Description of the Related Art
In response to recent needs for miniaturization of a reproducing device and a recording device, and increased recording capacity, a portable apparatus using a small-sized hard disk has been developed and commercialized.
For example, a hard disk is incorporated into portable apparatuses, notably a portable music player and a small-sized video camera, and contributes to realizing long-time reproduction and miniaturization of the apparatus.
On the other hand, as opportunities for carrying out such portable apparatuses including miniaturized devices have been increased, there are many cases where the apparatuses are damaged from accidental dropping or mishandling.
Japanese Patent Application Laid-Open No. 2003-263853 discusses a device which can prevent damages to a recording head and a disk face of a hard disk included in the device that is caused by accidental dropping or mishandling. The device prevents damages by retracting a head of the hard disk and stopping recording or reproducing in preparation for an impact coming from the fall of the apparatus.
The device discussed in Japanese Patent Application Laid-Open No. 2003-263853 uses an acceleration sensor to detect the drop.
The acceleration sensor includes a mass to detect gravity and a beam including a piezoelectric element to support the mass. However, such sensor has a specific resonant frequency determined by a structure including a mass and a beam.
When an external vibration agreeing with the resonant frequency of the acceleration sensor is applied to a device, the mass generates a resonant vibration and strongly vibrates. As a result, an output from the acceleration sensor increases.
Thus, when an external disturbance vibration of the resonant frequency is added to acceleration that is originally applied to an apparatus in the acceleration sensor, a detection amount of the sensor becomes greater than a detection amount of acceleration that is originally desired to be detected, under an influence of the resonant frequency. As a result, the accurate detection of acceleration cannot be performed.
FIG. 10 illustrates an example of frequency response of an acceleration sensor.
In FIG. 10, a horizontal axis in a graph indicates frequency, and a vertical axis indicates sensitivity.
When the characteristic of the graph is looked at, the output sensitivity is 0 dB which is proportional to acceleration up to about 400 Hz. However, the sensitivity gradually increases from about 400 Hz and has a peak greater than 20 dB at about 1.3 kHz. This remarkably high detection sensitivity is attributable to a resonance generated by the above-described structure of the sensor.
A vibration source causing a resonance is a driving vibration of an actuator such as a motor provided in the apparatus, a sound wave and vibration output from a speaker attached to the apparatus, vibration coming from an environment where the apparatus is used, and ambient sound.